1. Field of the Invention
The present invention relates to a photo detector. More particularly, the present invention relates to a wavelength-selective photo detector with an increased signal to noise (S/N) ratio.
2. Description of the Related Art
Photo detectors are used to detect an optical signal and convert the optical signal to an electrical signal having the same information as the optical signal. Photo detectors may be classified into categories such as detectors that use a pyro-electric effect, which enables photo detection by converting incident infrared rays into a voltage, semiconductor photo detectors that use generation of carriers in a semiconductor by optical absorption, or the like. The semiconductor photo detector may be a diode-type photo detecting device or a photoconductor-type photo detector. Generally, the semiconductor photo detector is formed using silicon and gallium arsenide (GaAs).
FIG. 1 illustrates a schematic conceptual diagram of an avalanche-type semiconductor photo detector.
Referring to FIG. 1, in a conventional avalanche photo detector, an i(Π) optical absorption layer 13 and a p-type amplification layer 15 are interposed between a p+-type electrode 11 and an n-type electrode 17. The p+-type electrode 11 is coupled to an external negative electrode and the n-type electrode 17 is coupled to an external positive electrode. Thus, the avalanche photo detector is driven by applying a strong reverse bias voltage to both ends of the avalanche photo detector.
FIG. 2 illustrates a graph showing the strength of an electric field in the avalanche photo detector where a reverse bias voltage is applied. Referring to FIG. 2, the strongest electric field is applied to the p-type amplification layer 15. The electric field affects movement rates of electrons and holes, which are generated in the optical absorption layer 13.
FIG. 3 illustrates a conceptual diagram showing the electron movement and the hole movement in the avalanche photo detector. Referring to FIG. 3, optical carriers, which are injected by a reverse bias voltage, absorb optical energy in the optical absorption layer 13. Thus, pairs of electrons and holes are generated and accelerated by the reverse bias voltage. The holes are accelerated toward the negative electrode and absorbed into the p+-type electrode 11, while the electrons are accelerated toward the positive electrode and sequentially collide with atoms of the p-type amplification layer 15 where a strong electric field is applied. Thus, secondary electrons are generated to amplify a current. This is called an “avalanche phenomenon.”
FIG. 4 illustrates a schematic circuit diagram of an equivalent circuit of the conventional avalanche photo detector as shown in FIG. 1. As shown in FIG. 4, in the conventional avalanche photo detector, noise current Inoise and signal current Isig flow through the equivalent circuit.
The avalanche photo detector can detect even an extremely feeble optical signal owing to the avalanche phenomenon. Also, a junction capacitance is small and a responsive characteristic is very good. However, the conventional avalanche photo detector leads not only to heat noise caused by a temperature increase, i.e, a Johnson-Nyquist noise, but also to shot noise caused by the flow of optical current having a wide bandwidth. Thus, the S/N ratio is degraded. To decrease the shot noise, both the bandwidth Δf of a received frequency and dark current of the photo detector should be reduced. Also, when a strong reverse bias voltage is applied to the photo detector, additional problems that are caused by the noise current need to be solved.